Transmission of a signal over a differential transmission line reduces the influence of noise or interference due to external stray electric fields. Any external signal sources tend to induce only a common mode signal on the transmission line and the balanced impedances to ground minimizes differential pickup due to stray electric fields. A differential transmission line allows a differential receiver to reduce the noise on a connection by rejecting common-mode interference. The transmission lines have the same impedance to ground, so the interfering fields or currents induce the same voltage in both wires. Use of such balanced circuits for differential signals, however, has generally been applied at lower frequencies.
A circuit element referred to as a balun is generally used to convert unbalanced transmission line inputs into one or more balanced transmission line outputs or vice versa. Baluns operating at low-frequency bands generally consist of a concentrated, constant component such as a transformer. Such low-frequency baluns often leverage ferrite and air coil transformer technology to achieve high performance and very broad bandwidth.
Trends in electronics, however, are generally toward ever increasing operational frequencies and bandwidths. Thus, baluns are being employed in various demanding applications often requiring high-frequency and/or wideband operation. For example, baluns are being incorporated in output stages of delta-sigma modulator direct digital synthesizers, Digital-to-Analog Converters (DACs), Analog-to-Digital Converters (ADCs), differential digital signaling, RF mixers, SAW filters, and antenna feeds. Such applications demand miniature, wide-bandwidth (wideband) baluns compatible with integrated circuits and capable of rejecting common mode energy from differential inputs or providing differential outputs lacking common mode energy.
At radio-wave frequencies (e.g., microwave) and higher it becomes increasingly difficult to fabricate broadband baluns having ferrite and air coil transformer, necessitating other techniques. Baluns that operate at such high-frequency bands generally consist of a distributed, constant component. Since most of these baluns each of which consists of a distributed, constant component include a quarter-wavelength matching element or are transformers whose size is determined according to usable wavelengths, a disadvantage to them is that their frequency bands are fundamentally narrow. Moreover, such high frequency signals (e.g., RF, microwave, millimeter wave) typically rely on single-ended and unbalanced anti-phase signals, rather than balanced differential signals. Namely, a signal is driven with reference to a ground. Such single-ended signals may be beneficial in controlling electromagnetic interference (consider high-frequency transmission lines, such as coaxial cable, in which an outer conductor is grounded). Unfortunately, such structures are not well suited to accommodate balanced differential signals, which are necessarily isolated from ground.